Selective electroacoustical transducer



July 28, 1942. E. M. SARGENT ETAL 2,291,108

SELECTIVE ELECTRO-ACOUSTICAL TRANSDUCER Filed Aug. 3, 1940 INVENTORSY EDWARD M. SARGENT LYNDON C. PAYMENT BY W MAM A T TORNE Y5.

Patented July 28, 1942 SELECTIVE ELECTROACOUSTICAL TRANSDUCER Edward M. Sargent and Lyndon C. Rayment,

Oakland, Calif.

Application August 3, 19 10, Serial No. 350,508

4 Claims.

This invention relates to electroacoustical apparatus for converting into audible sound electrical signals having a definite pitch, i. e., to an instrument of the loud speaker type having highly selective acoustical characteristics.

Owing to the present crowded condition. of communication channels for radio telegraphy a great deal of interference exists upon and between such channels, which is extremely difficult,

' if not-impossible, to eliminate even if the selectivity of the radio receiver used be so great as to make tuning difficult. This is particularly true uponthe bands used for amateur communications, although the situation aifects. all channels more or less. It will almost always be found that many signals can be heard simultaneously, no matter how selective the radio receiver used may be, and these signals may be'so nearly the same in intensity and pitch, or the pitches may be so nearly the same and the desired signals may be so faint in comparison with others which are audible, as to make reception extremely difficult. This difficulty is greatly increated if there is present a large amount of interferent noise or static.

The primary object of our invention is to make possible the separation of such signals from the interference due to both static and adjacent signalling channels. Pursuant to this broad object, secondary objects of this invention are to provide a means of separating desired signals from interference which may be applied to a radio receiver of any type without modification of the receiver itself; to provide a means of accomplishing this purpose which is highly effective as against interferent signals of definite pitch but is even more effective as against static; to provide a means of signal selection which is applied to the output rather than the input stages of aradio receiver; to provide a signal selector whose selectivity is under the control of the operator; to provide a device of the character described which may be utilized either as a loud speaker or with the equivalent of head-phones; and to provide a device for the purpose described which combines a high degree of efi'icacy with simplicity and low cost.

Other objects of our invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but we do not limit ourselves to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

The nature of the invention will beeapparent from the following description taken in connection with the accompanying drawing, wherein J Fig. 1 is a side elevation of one embodiment of the invention, the electrical circuit used in connection therewith being shown diagrammatically.

Fig. 2 is a plan view of the apparatus shown in Fig. 1.

Fig. 3 is an elevational view similar to that of Fig. 1 showing a somewhat different embodiment of the invention; and

Fig. 4 is a plan view of the device of Fig. 3.

Referring to Fig. 1 the device shown comprises a metal mounting disc or bed-plate I to which is clamped a permanent magnetic structure comprising a cross-yoke 2 and the magnet proper 3. The magnet is provided, as is customary in telephone receivers of the electromagnetic type, with a soft iron pole piece 4 upon which is mounted a spool 5 carrying a bobbin or reproducer coil 1.

Purely for structural reasons a sheet of insulating material 9 is positioned beneath the cross-yoke 2, and the yoke is clamped to the base plate by means of nuts I!) and washers H which are threaded onto four equally spaced screws l2 which serve to support as well the other mechanical parts of the device, and which are made of ferro-magnetic material to complete the circuit of the magnet. The exact form taken by the magnet and coil is relatively unimportant, and any structure which would suffice for the ordinary type of electromagnetic reproducer and which has sufficient electrical carrying capacity to support the current supplied by the output of the radio receiver used would also prove satisfactory here.

A diaphragm I3 of magnetic material in the form of the invention here described, is mounted in the field produced by the magnet 3 and closely adjacent the pole piece 4. The diaphragm is here shown as octagonal but may be circular or of other shapes if desired. It is supported upon the screws l2 which pass through holes adjacent the periphery of the diaphragm, nuts l4 and washers I5 being threaded upon these screws and supporting resilient thimbles l1. These thimbles may take various forms and may even be metallic, but I prefer to use soft or semi-soft rubber, it being understood that this term, as it is. here used, includes the various synthetic rubbers and rubber substitutes. Additional thimbles, washers, and nuts l1, l5 and M respectively are threaded on the screws above the diaphragm in inverted order to form clamps which hold the diaphragm loosely-in place, the adjustment of the screws being such that the rubber thimbles are substantially uncompressed and the diaphragm may be quite freely moved as a whole with the fingers.

It will be noted that in the drawing the diaphragm l3.appears to be quite thick, and this is the actual case in practice. The exact thickness used depends to some extent upon the elastic properties of the material employed, for the characteristic feature of this diaphragm is that it is tuned to a natural resonant period which is substantially that of the frequency chosen for selection by the device. To be satisfactory the diaphragm, if loosely held by its opposite edges between the fingers, or if supported as is shown in the drawing, and tapped quickly and lightly with the finger nail or a pencil, it should ring with the tone desired.

An acoustical resonator is mounted immediately above the diaphragm. In the present instance this resonator takes the form of a piece of metal tube to which brackets 2| are riveted. These brackets are engaged by the screws l2 and nuts 22, and the brackets are so positioned that the lower open end of the tube 20 is mounted about one-eighth of an inch above the diaphragm. This spacing is not in any sense critical, and rather Wide variations in the distance between the resonator and the diaphragm make little difference in the operation of the device. A second piece of tubing 23 telescopes the tube 20, and is used for tuning the resonator, being adjusted to such point that the resonant response of the air column is a maximum to the frequency chosen for selection, i. e., the natural frequency of the diaphragm. I

With the device as thus far described it will be found that if the leads 2t be connected directly to the output of a radio receiver of the heterodyne type, the musical note representing the signals, and whose pitch varies as the receiver is tuned, are reproduced with a volume closely approaching that of the ordinary loud speaker when, but only when, the pitch approaches the natural period of the diaphragm. For lower frequencies the diaphragm attempts to vibrate as a whole, and the restoring force of the loose thimbles i1 is so small that the resonant frequency of the system vibrating in this mode is practically below the limit of audibility. The loose clamps are so spaced, however, that they do not materially restrict the vibration of the diaphragm in accordance with its own natural mode of oper-' ation, and its response to signals of its own frequency is relatively large. The mass of the diaphragm itself prevents any great response to frequencies higher than its natural period.

The thickness and consequent large mass and inertia of the diaphragm contribute largely to this effect. It is possible to tune a thin diaphragm to the desired signal frequency, and as between otherwise interferent signals the thin diaphragm is nearly as effective as the thicker one. As against static, however, coming as sharp, irregular pulses or impacts the high inertia diaphragm is almost completely refractory, and its use results in an enormous reduction of static to signal ratio.

Except for the presence of the acoustic resonator, however, the acoustic impedance of the device would be such that the signal would be practically inaudible. The resonator, however, permits a large transfer of energy to the air at the selected frequency.

As thus far described, therefore, the device meets .most of the requirements for which it is designed, but there are two properties which may cause annoyance in its use. The first of these is that the response of the diaphragm may be so' sharp that the diaphragm continues to ring after the cessation of the signal, tending to bridge the spaces between the dots and dashes of which the. signal is composed and making it difllcult to read. This effect will not ensue if the loading imposed upon the diaphragm by the resonator be sumciently great, but in many cases othervfeatures of design may fix the degree of acoustic coupling between the diaphragm and the resonator and if the ringing occurs it may be advisable to regu late it by damping the diaphragm rather than changing the design as a whole. This may be readily accomplished by the application of the device indicated by the reference character 25, which is merely a small pieceof rubber slit to fit over the edge of the diaphragm and clamp it. The amount of damping thus provided is adjustable by positioning the damper, its eifect being least when it is placed on the diaphragm closely adjacent the supporting screws I2 and becoming greater as it is moved away therefrom. The device is most effective when the damping is such that there is no perceptible ring; further increase in damping reduces the intensity of the signal and again decreases its intelligibility. The selectivity of the device is thus under the control of the user, who may adjust it to suit his own ear.

The second of the two effects mentioned as disturbing occurs onlyv in the event of heavy static surges of low frequency. Such surges are not uncommon in the case of heavy interference,

and when they do occur they may be of such power as to move the diaphragm as a whole within its clamps, and cause it to stick against the magnetized pole piece. This may be completely eliminated by including in the circuit a condenser 26. Such a condenser will, howeveiyof itself increase the impedance of the circuit to the desired frequency and reduce the response thereto, and it is therefore desirable to time this circuit, either by the combination of the. condenser and reproducer coil 7 itself, or by the addition of further inductance 21. It is not desirable that this circuit be too sharply tuned, since this increases the number of degrees of freedom of the circuit as a whole and makes adjustment difficult in production. The fine winding of the reproducer coil and the acoustic impedance of the reproducer as a whole generally prevents this circuit from being too sharply tuned, but should this effect occur additional resistance may be introduced in the coil 21 which makes the response of the device less critical without noticeably reducing its effectiveness.

A modified form of the device shown in Figs. 3 and 4 differs only in detail from that described and the corresponding parts of the device, where they do not differ in function from those shown in the first figure, are designated by the same reference characters.

In this case, however, the motor element takes a slightly different form, an electromagnet being used instead of the permanent magnet 3. This electromagnet comprises a laminated core 3' formed of E-shaped punchings, and the coil 1' supplies both the signal and the magnetic bias for the device by carrying the plate current of the output tube to which it is connected.

' 1, however, in being of the closed type, wherein a pressure loop is formed at the closed end '3l. Where the device is used as a loud speaker this gives substantially no different results from those obtained from the type first described.

In this case, however, there is shown a tube 32 of relatively small bore which opens into the closed end 3| of the resonator. This tube is in practice used for attaching a stethoscope tube, and when so used the advantages of head-phone reception may be obtained in the exclusion of interferent local noises.

Since this particular form of the device obtains its magnetic bias from the plate circuit which feeds it, the series resonant circuit comprising the condenser 26 and the inductor 21 cannot be included directly in the line. They may equally well, as far as effect is concerned, be included in an audio circuit feeding the last tube, as is indicated purely schematically'by the showing of the grid 34 and plate 35.

It should not be understood that the distinctive features of the forms here shown are necessarily combined in the manner disclosed. Permanent magnet motors may be utilized with closed end resonators, and electromagnetic motors with ,open end resonators, nor is it necessary that the magnetic diaphragm type of motor be used, as thecharacteristic feature is the heavy, tuned diaphragm, which may be driven in any manner known to the art. Furthermore, it is not necessary that closed end resonators be used where a stethoscopic connection is desired, as long as the connection. be made at or near a loop of pressure. The varying forms are mere- Ly shown to give some indication of the scope We claim:

1. A selective electro-acoustical transducer consisting of a base, a plurality of parallel circularly arranged supports extending from said base, a relatively stiff disc diaphragm resiliently and loosely held at spaced points-on the periphery thereof by said supports in a position parallel to said base, said supports having portions extending beyond said diaphragm, a magnetic core carried by said base extending toward and short of said diaphragm, a reproducer coil mounted on said core, and an acoustic resonator mounted on the portions of said supports extending beyond said diaphragm, and having an open end positioned over, parallel to and spaced from said diaphragm inside of said supports.

2. A selective electro-acousticical transducer consisting of a base, a plurality of parallel circularly arranged supports extending from said base, a relatively stiff disc diaphragm resiliently and loosely held at spaced points on the periphery thereof by said supports in a position parallel to said base, said supports having portions extending beyond said diaphragm, a magnetic core carried by said base extending toward and short of said diaphragm, a reproducer coil mounted on said core, and a resonator cylinder having an open end, said cylinder being mounted on the portions of said supports extending beyond said diaphragm with said open end adjacent and parallel to the surface of said diaphragm inside of said supports, said cylinder being formed by two telescoping sections thereof.

3.- Apparatus in accordance with claim 2 wherein members of resilient material are attached to the periphery of said diaphragm between the points thereof held by said supports.

4. Apparatus in accordance with claim 2 wherein members of resilient material are attached to the periphery of said diaphragm between the points thereof held by said supports, said members being movable along said periphery and frictionally held thereon.

EDWARD M. SARGENT. LYNDON C. RAYMENI'. 

